DESCRIPTION: The objective of this work is to understand the mechanism of thin filament regulation, with particular emphasis on the role of tropomyosin. Activation of contraction in muscle and non-muscle cells by Ca2+ and myosin crossbridges in highly cooperative. Tropomyosin in striated muscles, along with troponin, is positioned along the length of the actin filament, and is responsible for this cooperativity. Contractile function, best understood in the context of muscle contraction, is also involved in many basic cellular functions including cell division, cell migration, and cell shape changes. Mutations in tropomyosin and troponin T have been found in patients with familial hypertrophic cardiomyopathy. Changes in tropomyosin isoform expression are associated with viral transformation and breast cancer. The proposal focuses on a set of hypotheses concerning the basic structural features of tropomyosin required for function. Recombinant proteins will be studied using a variety of functional assays: ability to form a coiled coil (determined using circular dichroism), actin binding, troponin binding (using a biosensor assay to be developed), cooperative interaction with myosin, and regulation of the actomyosin ATPase. Synthetic peptides will be used for structural analyses using NMR and CD.